Solvent-free and low-solvent two-component polyurethane lacquers have acquired increasing significance over recent years. Two-component lacquers based on 4,4'-diisocyanato diphenyl methane are ideally suitable for quick-hardening coatings on concrete, steel, highly stressed industrial floors, ships decks and fuel oil containers, and are being used to an ever increasing extent. Such lacquers have the considerable advantage that they harden extremely quickly and lead more quickly to the expected properties at room temperature than is possible with any other known 2-component polyurethane lacquer. They have the additional advantage of a relatively low vapor pressure.
Nevertheless, conventional products are also attended by considerable disadvantages:
4,4'-diphenyl methane diisocyanate may only be used as a starting product for polysolvent-free lacquers because the pure compound crystallizes at 37.degree. C. and, when stored, dimerizes into a solid substance. The starting product used contains polyfunctional homologue mixtures having a functionality of greater than 2. Although its tendency towards crystallization is minimal, it is, on the other hand, very dark in color, with the result that it cannot be used for light-colored lacquers. In addition, the viscosity of this starting product is relatively high, which makes it difficult to process, for example, into solvent-free spray lacquers. Another disadvantage is that these starting polyisocyanates often show only limited compatibility with various reactants, such as hydroxy polyethers. When the components are mixed, hazy mixtures are initially formed and only become clear and homogeneous, if at all, after a certain preliminary reaction.
The pot-life of the mixture of the two components is also an important factor. The known two-component systems which are based on 4,4'-diphenyl methane diisocyanate and contain less than 20% of 2,4'-isomer have only a short pot-life and, hence, a limited processing time, with the result that it is only possible to make up relatively small batches (cf. Hans Wagner/Hans Friederich Sarx, Lackkunstharze, Carl Hanser Verlag, 5th Edition, Munich, 1971, inter alia pages 158 to 170; Hans Knittel, Lehrbuch der Lacke und Beschichtungen, Vol 1, part 2, Verlag W. A. Colomb in H. Hennemann GmbH, BerlinOberschwandorf, 1973, pages 586 eq seq).
Accordingly, there is a need for solvent-free and low-solvent polyurethane lacquers for quick-hardening coatings which show improved compatibility, lead to longer pot-lives, the polyisocyanate component of which does not have a high vapor pressure and does not show any tendency towards the development of deposits and crystallization, are light in color and have none of the above-mentioned disadvantages of conventional two-component polyurethane lacquers.
The present invention provides a new lacquer system which satisfies the above requirements. It has been found that two-component polyurethane lacquers having the requisite properties may be obtained by using as the polyisocyanate component certain reaction products defined hereinafter of certain selected polyisocyanates of the diphenyl methane series with certain selected polyhydroxy polyethers.